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Summary of Significant Accounting Policies - Additional Information (Detail)
$ / shares in Units, $ in Millions
 1 Months Ended 12 Months Ended
Jan. 31, 2023
$ / shares
Aug. 31, 2022
Dec. 31, 2022
USD ($)
Subsidiary
mi
$ / shares
Dec. 31, 2021
USD ($)
$ / shares
Dec. 31, 2020
USD ($)
$ / shares
Dec. 31, 2019
USD ($)
Significant Accounting Policies [Line Items]            
Length Of Pipeline | mi     86      
Cost of removal obligation     $ 116.1 $ 107.5    
Regulatory assets     114.3 156.3    
Investments in trading securities     $ 5.8 5.7    
Percentage of total sales volumes revenue subject to RDM   43.00%        
Number of Subsidiaries | Subsidiary     3      
Allowance for doubtful accounts     $ 2.6 $ 3.3 $ 3.3 $ 1.0
Depreciation rate based on average depreciable property balance     3.26% 3.29% 3.34%  
Dividends per Common Share | $ / shares     $ 1.56 $ 1.52 $ 1.50  
Increase in dividend declared amount per share | $ / shares     $ 1.56      
Weighted average cost inventory amount     $ 1.8 $ 1.0    
Average Interest Rate On Debt     2.50% 1.71% 3.12%  
Other Deferred Charges            
Significant Accounting Policies [Line Items]            
Regulatory assets     $ 11.1 $ 15.3    
Materials And Supplies            
Significant Accounting Policies [Line Items]            
Weighted average cost inventory amount     11.4 8.6    
Subsequent Event            
Significant Accounting Policies [Line Items]            
Increase in dividend declared amount per share | $ / shares $ 1.62          
Unbilled Revenues            
Significant Accounting Policies [Line Items]            
Allowance for doubtful accounts     0.1 0.2    
Accounts Receivable            
Significant Accounting Policies [Line Items]            
Allowance for doubtful accounts     $ 2.5 3.1    
Utilities            
Significant Accounting Policies [Line Items]            
Number of Subsidiaries | Subsidiary     3      
Unitil Service; Unitil Realty; and Unitil Resources            
Significant Accounting Policies [Line Items]            
Number of Subsidiaries | Subsidiary     3      
Fitchburg Gas and Electric Light Company | Electric and Gas Division | Other Deferred Charges            
Significant Accounting Policies [Line Items]            
Hardship accounts in regulatory assets     $ 5.8 $ 7.9    
Quarterly Payment            
Significant Accounting Policies [Line Items]            
Dividends per Common Share | $ / shares     $ 0.39 $ 0.38 $ 0.375  
Quarterly Payment | Subsequent Event            
Significant Accounting Policies [Line Items]            
Common stock dividend paid per share | $ / shares 0.405          
Increase in dividend declared amount per share | $ / shares $ 0.015          
Environmental and Rate Case Costs and Other Expenditures | Recovered over the next seven years            
Significant Accounting Policies [Line Items]            
Regulatory assets     $ 7.2      
ISO-NE Obligations            
Significant Accounting Policies [Line Items]            
Cash Deposits     $ 6.0 $ 2.7    
Maximum            
Significant Accounting Policies [Line Items]            
Lease term     12 months      
Deferred Compensation Plan [Member]            
Significant Accounting Policies [Line Items]            
Investments in trading securities     $ 0.6 $ 0.6